The present invention relates to an electrical matrix keyboard scanning circuit, especially to a scanning circuit for an electrical matrix keyboard having extended keys.
An electrical keyboard is extensively used as an user input interface in computers, telephones, etc. A circuit coupled to the keyboard detects which key is pressed by positive-logic scanning or negative-logic scanning. For instance, as shown in FIG. 1a, the keys of a keyboard are arranged matrix-like, where R1 to R4 correspond row 1 to row 4, and C1 to C4 correspond column 1 to column 4. When a key is pressed, its corresponding row line (e. g. R2) and column line (e. g. C3) are connected. In a negative-logic scanning circuit, row lines are firstly set to high voltage level with high impedance, and column lines are set to low voltage with low impedance. When a row line is connected to a column line, its voltage level will drop to the low level. The states of the row lines are stored in a latch circuit. After that, column lines are set to the high voltage level with high impedance, and row lines are set to the low voltage with low impedance. When the column line is connected to the row line as stated above, its voltage level drops to low level, and the states of the column line are stored in another latch circuit. By reading out the states stored in the latch circuits, the position of the pressed key can be easily determined. The steps described above are typically repeated for several times to confirm that the detected key has been intentionally pressed by user.
The keyboard shown in FIG. 1a is a typical 4.times.4 matrix keyboard. One of the integrated circuits (ICs) designed for scanning such a keyboard is the UM91230C chip made by United Microelectronics Corp. (UMC), which provides eight pins or ports for scanning, R1 to R4 and C1 to C4. Thus, the keyboard can only have 16 input keys. However, more keys are often required in modern applications, and number of pins for the keyboard scanning circuit IC increases as the number of keys which may be scanned increases. Increasing the number of pins raises the cost of manufacture of the I. C.